Design rules for integrated circuit fabrication are becoming increasingly fine. Design rules of 0.5 .mu.m are being replaced by design rules that are less than 0.5 .mu.m. These increasingly fine design rules require processes which can delineate features in the integrated circuit with the required accuracy.
Lithographic processes are used during the fabrication of integrated circuits. A lithographic process employs energy that is introduced onto selected portions of an energy sensitive resist material (imaging layer) overlying a substrate. One way in which energy is introduced into selected portions of the resist is through openings in a mask substrate interposed between the energy source and the resist material. These openings in the mask substrate define the pattern. The pattern is transferred into the resist material by the energy that is permitted to pass through the openings in the mask substrate and into the resist. Thus, it is an image of the pattern defined by the mask substrate that is transferred into the resist material.
After the image is transferred into the resist material, the resist material is developed to form a pattern. The pattern is then transferred by etching into the substrate underlying the resist material. Once the pattern is incorporated into the substrate, it becomes a feature of the integrated circuit.
The energy used to expose the resist material, the composition of the resist material, the thickness of the resist material, and many other factors affect the ability of a lithographic process to delineate a feature in a substrate. The smaller the design rule, the more precisely the feature must be delineated.
Current lithographic processes use solution-developed resist materials. However, as design rules decrease to 0.25 .mu.m, 0.18 .mu.m, and smaller, lithographic processes that use dry-developed resists are becoming more attractive. Although lithographic processes that use dry-developed resists usually have more processing steps, these processes offer certain advantages when used for fabricating devices subject to these smaller design rules. These advantages include minimized linewidth variations over topography and enhanced depth of focus. Processes that utilize dry-developed resists offer these advantages because the image is transferred into the surface of the imaging layer, not throughout its entire thickness. Thus, the active region in which the image is focussed is not necessarily the entire thickness of the imaging layer. A thinner lithographically active region is advantageous because it is easier to precisely introduce the image into a thinner region.
Processes that use these dry-developed resists are referred to as surface-imaging lithographic processes. The processes are so named because they permit the image to be introduced near the surface of the resist. Surface-imaging lithographic processes also provide the promise of higher resolution patterns and the elimination of the need for an antireflective coating.
Because the dry-developed resists have a thinner lithographically active region, they must also have greater development selectivity between the exposed and unexposed regions of the resist layer than thicker conventional resists. Consequently, surface-imaging lithographic processes which introduce the requisite etch selectivity into the dry-developed resists used in these processes are being investigated.